Test object for detecting aberrations of an optical imaging system

ABSTRACT

Aberrations of an imaging system (PL) can be detected in an accurate and reliable way by imaging, by means of the imaging system, a circular phase structure ( 22 ) on a photoresist (PR), developing the resist and scanning it with a scanning detection device (SEM) which is coupled to an image processor (IP). The circular phase structure is imaged in a ring structure ( 25 ) and each of several possible aberrations, like coma, astigmatism, three-point aberration, etc. causes a specific change in the shape of the inner contour (CI) and the outer contour (CE) of the ring and/or a change in the distance between these contours, so that the aberrations can be detected independently of each other.  
     The new method may be used for measuring a projection system for a lithographic projection apparatus.

[0001] The invention relates to a method of detecting aberrations of anoptical imaging system, comprising the steps of:

[0002] arranging a test object in the object plane of the system;

[0003] providing a photoresist layer in the image plane of the system;

[0004] imaging the test object by means of the system and an imagingbeam;

[0005] developing the photoresist layer, and

[0006] detecting the developed image by means of a scanning detectiondevice having a resolution which is considerably larger than that of theimaging system.

[0007] The fact that the resolution of the scanning detection device isconsiderably larger than that of the imaging system means that thedetection device allows observation of details which are considerablysmaller than the details that can still be separately imaged by theimaging system.

[0008] An optical imaging system in the form of a projection lens systemhaving a large number of lens elements is used in photolithographicprojection apparatuses which are known as wafer steppers or as waferstep-and-scanners. Such apparatuses are used, inter alia, formanufacturing integrated circuits, or ICs. In a photolithographicprojection apparatus, a mask pattern present in the mask is imaged alarge number of times, each time on a different area (IC area) of thesubstrate by means of a projection beam having a wavelength of, forexample, 365 nm in the UV range, or a wavelength of, for example, 248 nmin the deep UV range, and by means of the projection lens system.

[0009] The method mentioned above is known from the opening paragraph ofEP-A 0 849 638, relating to a method of measuring the comatic aberrationof projection lens systems in lithographic projection apparatuses.

[0010] The aim is to integrate an ever-increasing number of electroniccomponents in an IC. To realize this, it is desirable to increase thesurface area of an IC and to decrease the size of the components. Forthe projection lens system, this means that both the image field and theresolution must be increased, so that increasingly smaller details, orline widths, can be imaged in a well-defined way in an increasinglylarger image field. This requires a projection lens system which mustcomply with very stringent quality requirements. Despite the great carewith which such a projection lens system has been designed and the greatextent of accuracy with which the system is manufactured, such a systemmay still exhibit aberrations such as spherical aberration, coma andastigmatism which are not admissible for the envisaged application. Inpractice, a lithographic projection lens system is thus not an ideal,diffraction-limited system but an aberration-limited system. Saidaberrations are dependent on the positions in the image field and are animportant source of variations of the imaged line widths occurringacross the image field. When novel techniques are used to enhance theresolving power, or the resolution, of a lithographic projectionapparatus, such as the use of phase-shifting masks, as described in, forexample, U.S. Pat. No. 5,217,831, or when applying an off-axisillumination as described in, for example, U.S. Pat. No. 5,367,404, theinfluence of the aberrations on the imaged line widths still increases.

[0011] Moreover, the aberrations are not constant in modern lithographicprojection lens systems. To minimize low-order aberrations, such asdistortion, curvature of the field, astigmatism, coma and sphericalaberration, these systems comprise one or more movable lens elements.The wavelength of the projection beam or the height of the mask tablemay be adjustable for the same purpose. When these adjusting facilitiesare used, other and smaller aberrations are introduced. Moreover, sincethe intensity of the projection beam must be as large as possible,lithographic projection lens systems are subject to aging so that theextent of the aberrations may change with respect to time.

[0012] Based on the considerations described above, there is anincreasing need for a reliable and accurate method of measuringaberrations.

[0013] It has also been proposed to use for the projection beam a beamof extreme UV (EUV) radiation, i.e. radiation at a wavelength in therange of several nm to several tens of nm. The resolution of theprojection lens system can thereby be enhanced considerably withoutincreasing the numerical aperture (NA) of the system. Since no suitablelens material is available for EUV radiation, a mirror projection systeminstead of a lens projection system must then be used. A lithographicmirror projection system is described in, inter alia, EP-A 0 779 258.For reasons analogous to those for the lens projection system, there isa need for an accurate and reliable method of measuring aberrations forthis EUV mirror projection system as well.

[0014] The opening paragraph of said EP-A 0 849 638 rejects the methodin which the image of a test mask formed in the photoresist layer isscanned with a scanning detection device in the form of a scanningelectron microscope. Instead, it is proposed to detect said image withoptical means. To this end, a test mask having one or more patterns ofstrips which are alternately radiation-transmissive andradiation-obstructive, i.e. an amplitude structure, is used. The comaticaberration of a projection system can be detected with such a pattern.The detection is based on measuring the widths of the light or darkstrips in the image formed and/or measuring the asymmetry between thestrips at the ends of the image of the patterns.

[0015] It is an object of the present invention to provide a method ofthe type described in the opening paragraph, which is based on adifferent principle and with which different aberrations can be measuredindependently of each other. This method is characterized in that use ismade of a test object which comprises at least one closed single figurehaving a phase structure, and in that the image of this figure observedby the scanning detection device is subjected to an image analysis inorder to ascertain at least one of different types of changes of shapein the image of the single figure, each type of shape change beingindicative of a given kind of aberration.

[0016] A single figure is understood to mean a figure having a singlecontour line which is closed in itself. The contour line is the boundaryline between the figure and its ambience.

[0017] The invention is based on the recognition that the contour lineof a figure having a phase structure is not imaged in a single line butin a first and a second image line, the second image line being locatedwithin the first image line, and the distance between the first and thesecond image line is determined by the point spread function, or Airydistribution, of the imaging system. In the method according to theinvention, useful use is thus made of the point spread function, or Airydistribution, of the imaging system. If this system has givenaberrations, given deviations of the ideal image occur, such asdeviations of the shape of the image lines themselves and/or changes ofthe mutual position of the two image lines. The novel method thus allowsdetection of aberrations which cannot be detected when using a testobject in the form of an amplitude, or black-white, structure. Whenusing a test object with an amplitude structure, its contour line isimaged in a single line. Consequently, only the aberrations of theimaging system which cause deviations of the imaged single contour linecan be detected when using such a test object, and this even lessaccurately. When using a test object having a phase structure, differentaberrations occurring simultaneously can be detected separately becausethe effects of the different aberrations remain well distinguishable inthe image formed, in other words, the different aberrations do notexhibit any mutual crosstalk.

[0018] It is to be noted that, in one embodiment described in U.S. Pat.No. 5,754,299, relating to a method and a device for measuring anasymmetrical aberration of a lithographic projection system, the testobject is denoted as phase pattern. However, this pattern is not aclosed single figure, but a phase grating, for example, an alignmentmark. The image formed of this grating has the same appearance as thegrating itself, i.e. each grating line is imaged in a single line.Moreover, for measuring the aberration, an image of the grating isformed every time at different focus settings, and the detection isbased on measuring the asymmetries between these images, rather than ondetecting changes of shape and/or positions in an image itself.

[0019] The method is further preferably characterized in that a scanningelectron microscope is used as a scanning detection device.

[0020] Such a microscope which is already frequently used in practicehas a sufficient resolution for this application. Another and newer typeof scanning detection device is the scanning probe microscope which isavailable in several implementations such as the atomic force microscope(AFM) and the scanning optical probe microscope.

[0021] The method is further preferably also characterized in that theimage analysis method used comprises a Fourier analysis.

[0022] Since the Fourier analysis operates with sine and cosinefunctions, it is eminently suitable to directly analyze the contourlines of the image.

[0023] The phase structure of the test object may be realized in variousways. For example, the single figure may be constituted by an area in atransparent plate having a refractive index which is different from thatof the rest of the plate.

[0024] A preferred embodiment of the novel method is characterized inthat every single figure is constituted by an area in a plate located ata different height than the rest of said plate.

[0025] Said area may be countersunk in the plate or project from theplate. This plate may be transparent to the radiation of the imagingbeam, or reflective.

[0026] The single figure may have various shapes, such as the shape ofsquare or of a triangle. A preferred embodiment of the novel method ischaracterized in that said area is circularly shaped.

[0027] The shape of the single figure is then optimally adapted to thecircular symmetry of the imaging system, and the image of this figureconsists of two circular image lines. A change of the shape and a mutualoffset of these image lines can be observed easily. Even if a squaresingle figure is used, the novel method yields good results because theimage lines of this figure formed by the projection system are asufficient approximation of the circular shape.

[0028] Each single figure is preferably further characterized in thatthe height difference between the area of this figure and the rest ofthe plate is such that a phase difference of 180° is introduced in theimaging beam.

[0029] For a transmissive, or reflective, test object, this means thatthe height difference must be of the order of λ/(2(n₂−n₁)), or of λ/4n,in which λ is the wavelength of the imaging beam, n₂ is the refractiveindex of the material of the test object and n₁ is the refractive indexof the surrounding medium. At this height difference, the phasedifference between the part of the imaging beam originating from thearea of the single figure and the part of the imaging beam originatingfrom the surroundings of this area is maximal, and the contrast in theimage formed is maximal. If the diameter of the area is of the order ofthe wavelength of the imaging beam, or of a larger order, the optimalheight difference is equal to λ/(2(n₂−n₁)) or λ/4n. At a smallerdiameter, polarization effects must be taken into account, and theoptimal height difference deviates by several percent from thelast-mentioned values.

[0030] In accordance with a further preferred embodiment, the diameterof the area is proportional to λ/(NA.M), in which λ is the wavelength ofthe imaging beam, NA is the numerical aperture of the projection systemat the image side and M is the magnification of this system.

[0031] The size of the test object is then adapted to the resolution ofthe projection system, allowing measurements of aberrations of thesmallest images that can be made with the projection system.

[0032] The method may be used, inter alia, for detecting aberrations ofa projection system in a lithographic apparatus intended to image a maskpattern, present in a production mask, on a production substrate whichis provided with a photoresist layer. This method is furthercharacterized in that a mask having at least a single figure with aphase structure is used as a test object, which mask is arranged at theposition of a production mask in the projection apparatus, and in that aphotoresist layer with a support is provided at the position of aproduction substrate.

[0033] This method provides the advantage that aberrations of theprojection system can be detected under circumstances which correspondto those for which this projection system is intended. The number ofsingle figures may vary from one to several tens. Since these figuresare imaged at different positions within the image field of theprojection system, insight is obtained into the variations of theaberrations across the image field. Since the single figures are small,they may be provided in the production mask at positions outside thedetails of the mask pattern.

[0034] However, the method is preferably further characterized in thatuse is made of an empty test mask having at least a single figure.

[0035] The test object is now constituted by a recessed or a raised partof a transparent plate of the same material and having the samethickness as a production mask, but without a mask pattern or partsthereof, which plate may be denoted as empty test mask.

[0036] The invention further relates to a system for performing themethod described above. The system comprises an optical apparatus ofwhich the imaging system forms part, a test object having at least asingle figure with a phase structure, a scanning detection device forscanning at least a test object image formed by the imaging system, andan image processor, coupled to the scanning projection device, forstoring and analyzing the observed images, and is characterized in thatthe image processor comprises analysis means for detecting at least oneof different types of changes of the shape of said image.

[0037] The invention also relates to a lithographic projection apparatusfor imaging a mask pattern, present in a mask, on a substrate, whichapparatus comprises an illumination unit for supplying a projectionbeam, a mask holder for accommodating the mask, a substrate holder foraccommodating the substrate, and a projection system arranged betweenthe mask holder and the substrate holder, which apparatus is suitablefor performing the method described above. This apparatus ischaracterized in that, in the implementation of the method, theprojection beam is used as an imaging beam, and in that the illuminationunit comprises means for reducing the diameter of the projection beamcross-section for the method to a value which is smaller than thediameter of the projection beam cross-section during projection of themask pattern on the substrate.

[0038] The invention further relates to a test object for use with themethod described above. This test object has one or more of thecharacteristic features of the above-mentioned embodiments of the methodrelating to the test object.

[0039] This test object may further have the characteristic features asclaimed in claims 12-17.

[0040] These and other aspects of the invention are apparent from andwill be elucidated, by way of non-limitative example, with reference tothe embodiments described hereinafter.

[0041] In the drawings:

[0042]FIG. 1 shows diagrammatically an embodiment of a photolithographicprojection apparatus with which the method can be performed;

[0043]FIG. 2 is a block diagram of the system for performing the method;

[0044]FIG. 3a is a bottom view of a test object with a single figure inthe form of a recess;

[0045]FIG. 3b is a cross-section of this test object;

[0046]FIG. 4 shows the annular image formed of said recess;

[0047]FIG. 5 shows the theory of the image formation;

[0048]FIG. 6 shows an annular image without aberrations;

[0049]FIG. 7 shows an annular image with coma;

[0050]FIG. 8 shows an annular image with astigmatism;

[0051]FIG. 9 shows an annular image with three-point aberration;

[0052]FIG. 10 shows the variation of the ring width of an annular imagewith spherical aberrations for different focus settings;

[0053]FIG. 11 shows this variation in a graphic form;

[0054]FIG. 12 shows an annular image picked up under the best focuscondition;

[0055]FIG. 13 shows the different Fourier terms associated with thisimage;

[0056]FIG. 14 shows the variation of a spherical aberration across theimage field of the projection system;

[0057]FIG. 15 shows annular images with coma formed at differentpositions in the image field;

[0058]FIG. 16 shows such an image on a larger scale, formed at an angleof the image field;

[0059]FIG. 17 shows the different Fourier terms associated with thisimage;

[0060]FIG. 18 shows a chart of the coma measured at 21 positions in theimage field;

[0061]FIG. 19 shows annular images with astigmatism formed at differentpositions in the image field;

[0062]FIG. 20 shows such an image on a larger scale, formed at an angleof the image field;

[0063]FIG. 21 shows the different Fourier terms associated with thisimage;

[0064]FIG. 22 shows a chart of the astigmatism measured at 21 positionsin the image field;

[0065]FIG. 23 shows the variation of a three-point aberration across theimage field of the projection system;

[0066]FIG. 24 shows the influence of spherical aberration andastigmatism on the measured coma across the image field;

[0067]FIG. 25 shows the influence of spherical aberration and coma onthe measured astigmatism across the image field;

[0068]FIG. 26 shows a small part of an embodiment of a test mask with adetection mark and a further mark, and

[0069]FIG. 27 shows an embodiment of a lithographic projection apparatuswith a mirror projection system.

[0070]FIG. 1 only shows diagrammatically the most important opticalelements of an embodiment of a lithographic apparatus for repetitivelyimaging a mask pattern on a substrate. This apparatus comprises aprojection column accommodating a projection lens system PL. Arrangedabove this system is a mask holder MH for accommodating a mask MA inwhich the mask pattern C, for example, an IC pattern to be imaged isprovided. The mask holder is present in a mask table MT. A substratetable WT is arranged under the projection lens system PL in theprojection column. This substrate table supports the substrate holder WHfor accommodating a substrate W, for example, a semiconductor substrate,also referred to as wafer. This substrate is provided with aradiation-sensitive layer PR, for example a photoresist layer, on whichthe mask pattern must be imaged a number of times, each time in adifferent IC area Wd. The substrate table is movable in the X and Ydirections so that, after imaging the mask pattern on an IC area, asubsequent IC area can be positioned under the mask pattern.

[0071] The apparatus further comprises an illumination system which isprovided with a radiation source LA, for example, a krypton-fluorideexcimer laser or a mercury lamp, a lens system LS, a reflector RE and acondenser lens CO. The projection beam PB supplied by the illuminationsystem illuminates the mask pattern C. This pattern is imaged by theprojection lens system PL on an IC area of the substrate W. Theillumination system may be implemented as described in EP-A 0 658 810.The projection system has, for example, a magnification M=¼, a numericalaperture NA=0.6 and a diffraction-limited image field with a diameter of22 mm.

[0072] The apparatus is further provided with a plurality of measuringsystems, namely an alignment system for aligning the mask MA and thesubstrate W with respect to each other in the XY plane, aninterferometer system for determining the X and Y positions and theorientation of the substrate holder and hence of the substrate, and afocus error detection system for determining a deviation between thefocal or image plane of the projection lens system PL and the surface ofthe photoresist layer PR on the substrate W. These measuring systems areparts of servosystems which comprise electronic signal-processing andcontrol circuits and drivers, or actuators, with which the position andorientation of the substrate and the focusing can be corrected withreference to the signals supplied by the measuring systems.

[0073] The alignment system uses two alignment marks M₁ and M₂ in themask MA, denoted in the top right part of FIG. 1. These marks preferablyconsist of diffraction gratings but may be alternatively constituted byother marks such as squares or strips which are optically different fromtheir surroundings. The alignment marks are preferably two-dimensional,i.e. they extend in two mutually perpendicular directions, the X and Ydirections in FIG. 1. The substrate W has at least two alignment marks,preferably also two-dimensional diffraction gratings, two of which, P₁and P₂, are shown in FIG. 1. The marks P₁ and P₂ are located outside thearea of the substrate W where the images of the pattern C must beformed. The grating marks P₁ and P₂ are preferably implemented as phasegratings, and the grating marks M₁ and M₂ are preferably implemented asamplitude gratings. The alignment system may be a double alignmentsystem in which two alignment beams b and b′ are used for imaging thesubstrate alignment mark P₂ and the mask alignment mark M₂, or thesubstrate alignment mark P₁ and the mask alignment mark M₁ on eachother. After they have passed the alignment system, the alignment beamsare received by a radiation-sensitive detector 13, or 13′, whichconverts the relevant beam into an electric signal which is indicativeof the extent to which the substrate marks are aligned with respect tothe mask marks, and thus the substrate is aligned with respect to themask. A double alignment system is described in U.S. Pat. No. 4,778,275which is referred to for further details of this system.

[0074] For an accurate determination of the X and Y positions of thesubstrate, a lithographic apparatus is provided with a multi-axisinterferometer system which is diagrammatically shown by way of theblock IF in FIG. 1. A two-axis interferometer system is described inU.S. Pat. No. 4,251,160, and a three-axis system is described in U.S.Pat. No. 4,737,823. A five-axis interferometer system is described inEP-A 0 498 499, with which both the displacements of the substrate alongthe X and Y axes and the rotation about the Z axis and the tilts aboutthe X and Y axes can be measured very accurately.

[0075] A step-and-scan lithographic apparatus does not only comprise asubstrate interferometer system but also a mask interferometer system.

[0076] As is diagrammatically shown in FIG. 1, the output signal Si ofthe interferometer system and the signals S₁₃ and S₁₃′ of the alignmentsystem are applied to a signal-processing unit SPU, for example, amicrocomputer which processes said signals to control signals SAC for anactuator AC with which the substrate holder is moved, via the substratetable WT, in the XY plane.

[0077] The projection apparatus further comprises a focus errordetection device, not shown in FIG. 1, for detecting a deviation betweenthe focal plane and the projection lens system PL and the plane of thephotoresist layer PR. Such a deviation may be corrected by moving, forexample, the lens system and the substrate with respect to each other inthe Z direction or by moving one or more lens elements of the projectionlens system in the Z direction. Such a detection device which may besecured, for example, to the projection lens system, is described inU.S. Pat. No. 4,356,392. A detection device with which both a focuserror and a local tilt of the substrate can be detected is described inUS-A 5,191,200.

[0078] Very stringent requirements are imposed on the projection lenssystem. Details having a line width of, for example 0.35 μm or smallershould still be sharply imaged with this system, so that the system musthave a relatively large NA, for example, 0.6. Moreover, this system musthave a relatively large, well-corrected image field, for example, with adiameter of 23 mm. To be able to comply with these stringentrequirements, the projection lens system comprises a large number, forexample, tens of lens elements, and the lens elements must be made veryaccurately and the system must be assembled very accurately. A goodcontrol of the projection system is then indispensable, both fordetermining whether the system is sufficiently free from aberrations andis suitable to be built into the projection apparatus, and to be able toascertain whether aberrations may as yet occur due to all kinds ofcauses so that measures can be taken to compensate for theseaberrations.

[0079] For detecting the aberrations, the projection apparatus itselfmay be used as a part of a measuring system for performing a detectionmethod. In accordance with this method, a test mask having a given testpattern is arranged in the mask holder, and this test pattern is imagedin the radiation-sensitive, or photoresist, layer in the same way as aproduction mask pattern is imaged in the radiation-sensitive layerduring the production process. Subsequently, the substrate is removedfrom the apparatus and is developed and etched so that an image of thetest pattern in the form of a relief pattern in the substrate isobtained. This relief image is subsequently scanned by a scanningdetection device, for example, a scanning electron microscope. Theelectron microscope converts the observed image into image data whichare processed in an image processing device, using a special imageprocessing program. Its results may be visualized in diagrams or graphs.It is alternatively possible to show visual images of the structuresobserved by the electron microscope on, for example, a monitor.

[0080] This method is shown in a block diagram in FIG. 2. In thisFigure, the projection apparatus is denoted by PA, the developing andetching apparatus is denoted by ED, the electron microscope is denotedby SEM, the image processing device is denoted by IP and the monitor isdenoted by MO.

[0081] According to the invention, use is made of a test object with aphase structure, a small part of which test object is shown in a bottomview in FIG. 3a and in a cross-section in FIG. 3b. This test objectcomprises at least one closed figure with a phase structure in the formof a circular recess 22 in a transparent test mask of, for example,quartz. This recess has a diameter D and a depth d. Instead of a recess,a figure of the test object may be alternatively constituted by a raisedpart having the same diameter and the same height difference withrespect to the rest of the mask as said recess. Since the test mask issatisfactorily transparent to the projection beam with which the testfigure is imaged on the photoresist layer, this figure forms a phasestructure for this beam. This means that, after passage through the testmask, the part of the projection beam PB incident on the circular area22 has obtained a different phase than the rest of the beam. The phasedifference φ (in rad.) between the beam portions is defined by$\phi = \frac{{\left( {n_{2} - n_{1}} \right) \cdot d \cdot 2}\pi}{\lambda}$

[0082] in which n₂ is the refractive index of the mask material, n₁ isthe refractive index of the surrounding medium which is generally air,with n=1, and λ is the wavelength of the projection beam PB. The circle22 is imaged by the projection lens system in a ring 24 shown in FIG. 4.It can be explained with reference to FIG. 5 how this ring is obtained.

[0083] In this Figure, the reference numeral 22 denotes a circular areaof the test mask on which the projection beam PB, a beam ofelectromagnetic radiation, is incident. After passage through the phasepattern 22, the size of the electric field vector E of this beam showsthe variation as a function of the position p of graph 25. Theperpendicular slopes in this graph are located at the position of thecontour line of the phase pattern 22. After passage through theprojection lens system PL shown diagrammatically by means of a singlelens in FIG. 5, the size of the electric field vector E′ shows thevariation as a function of the position in graph 29.

[0084] The perpendicular slopes have changed to oblique slopes. This isa result of the fact that the projection lens system is not an idealsystem but has a point spread function, i.e. a point is not imaged as apoint but is more or less spread across an Airy pattern during imaging.If the projection system were ideal, the electric field vector wouldhave the variation as shown in the broken line graph 30. The size of theelectric field vector represents the amplitude of the projection beam,so that the graph 29 shows the amplitude of the beam as a function ofthe position in the plane of the photoresist layer PR. Since theintensity I of the beam is equal to the square of the amplitude (I=E′²),this intensity shows the variation as a function of the position ingraph 31. Each edge in the graph 29 has changed over to two edges withopposite slopes, which means that the single contour line of the phasepattern is imaged in two contour lines, i.e. the circle is imaged in aring 24 as shown in FIG. 4. The width wi of this ring is determined bythe point spread function and its diameter di is determined by theresolution of the projection lens system. If the projection lens systemdid not have any point spread, the intensity of the projection beam inthe photoresist layer would have the variation as shown by way of thebroken line graph 32, and the phase pattern 22 would be imaged as acircle. In the method according to the invention, deliberate use is madeof the point spread, though being small, of the projection lens system.

[0085] Upon use of this method in a given projection apparatus, the ring24 had a width wi of 80 nm and a diameter di of 350 nm. The projectionlens system had a magnification M=¼ that the phase pattern in the maskhad a diameter D=1.4 μm. The diameter di of 350 nm appeared to be anoptimal value and corresponded to the resolution of the apparatus whoseprojection lens system had an NA of 0.63 and the projection beam had awavelength of 248 nm. For other projection apparatus, di will have adifferent optimal value. Even if di has a value which is different fromthe optimal value, aberrations can still be measured.

[0086] For obtaining a good contrast in the image, the phase differencebetween the beam portion which has passed through the circular area 22and the rest of the beam must be φ=πrad. This means that the depth d ofthe recess must be equal to the wavelength of the beam PB if therefractive index of the mask material is 1.5 and the surrounding mediumis air having a refractive index of 1. For a practical embodiment, theoptimal depth d is, for example, 233 nm. Usable results can still beobtained at depths different from the optimal depth.

[0087] If use is made of a test mask in which both the circular area 22and its surroundings are reflecting, the optimal depth, or height, ofthe circular area is equal to a quarter of the wavelength.

[0088] If the projection lens system does not exhibit any aberrations,the inner circle ci and the outer circle ce of the ring in FIG. 4 areconcentric and, during scanning through focus, this ring has asymmetrical behavior. Scanning through focus is understood to mean themovement of the image along the optical axis of the projection lenssystem in the +Z direction and the −Z direction with respect to thephotoresist layer. This movement of the image with respect to the layercan be realized by changing the focus of the projection system or bymoving this system and the photoresist layer with respect to each otherin the Z direction.

[0089] When aberrations occur, said symmetrv is disturbed. Each kind ofaberration results in a characteristic deformation of the ring, as willbe explained hereinafter.

[0090] To be able to satisfactorily observe the inner circle ci and theouter circle ce which are located close together, a scanning microscopemay be used with a resolution which is larger than that (λ/NA) of theprojection system. A scanning electron microscope, which may have amagnification of the order of 100,000 and can observe details of theorder of 3.5 nm, is eminently suitable for this purpose, particularly ifa large number of images must be detected. It is alternatively possibleto use other scanning microscopes in the form of, for example, probemicroscopes such as an optical probe microscope or an AFM (Atomic ForceMicroscope) or hybrid forms thereof, particularly if only a small numberof images must be detected.

[0091] The image data obtained by scanning are subjected to a specialimage-processing method. This method may consist of, for example, twooperations. The first operation comprises a determination of thecontours of the ring in accordance with the steps of:

[0092] removing noise from the incoming image data;

[0093] determining the contours of the image, for example, bydifferentiation, or by determining in how far the intensity of eachobserved pixel is under a given threshold;

[0094] determining the point of gravity of the intensity distribution ofthe observed image;

[0095] measuring the distances between the pixels and this point ofgravity, and

[0096] plotting the measured distances in a histogram which then showstwo peaks, the inner edge of the peak, where the smaller distances areclustered, representing the inner contour of the ring, and the outeredge of the peak, where the larger distances are clustered, representingthe outer contour.

[0097] The second operation consists of a Fourier analysis comprisingthe steps of:

[0098] decomposing radii of these contours each time extending at adifferent angle Θ to the X axis into sine and cosine functions of theseangles, and filtering the contours, and

[0099] visualizing the intensities of the Fourier components thusobtained in graphs.

[0100] Analysis methods which are different from this Fourier analysismay be used instead. It is essential that the radii of the contours aremeasured as a function of the angle Θ. The advantage of the Fourieranalysis is that it has sine functions and cosine functions as basicfunctions. Determining the radii of the contours as a function of theangle Θ can be most easily done by way of the sine function and thecosine function. The aberrations can thereby be detected in a directmanner. More operations must be performed in other analysis methods.

[0101] If the projection lens system does not have any aberrations, theinner contour and the outer contour of the annular image aresatisfactorily circular, and the circles are satisfactorily concentricthroughout their circumference, as is shown in FIG. 6. Moreover, therotational symmetry is then maintained upon scanning through focus.

[0102]FIG. 6 shows a SEM image obtained by means of an aberration-freelens system and with the following imaging conditions: λ=248 nm,NA=0.63, σ=0.3, the thickness of the photoresist layer 280 nm. σ, alsoreferred to as the degree of coherence, indicates the extent to whichthe imaging beam fills the pupil of the lens system. A σ of 0.3 meansthat the imaging beam has a cross-section which is equal to 0.3 of thepupil cross-section.

[0103] The major aberrations of the projection lens system are coma,astigmatism, the three-point, or three-leaf, aberration, and sphericalaberration. If the projection lens system has coma, the image formedtherewith and observed by the SEM has the shape as shown in FIG. 7. Thecoma in this example is obtained artificially by deliberately changingthe wavelength of the imaging beam to some extent. The other imagingconditions are equal to those mentioned with reference to FIG. 6. Theimage formed in FIG. 7 is an image formed in the top right angle of theimage plane if this image plane is considered to coincide with the planeof the drawing, likewise as in FIG. 15 to be described hereinafter. Thisimage has a coma of 45°. The inner contour and the outer contour arecircles which are no longer centered with respect to each other but areoffset with respect to each other in the direction of the coma, hence inthe direction of 45°.

[0104]FIG. 8 shows an image formed by a projection lens system havingastigmatism. The other imaging conditions are again equal to thosementioned with reference to FIG. 6. The contour lines of the astigmaticimage are elliptical, while the distance between these lines, i.e. thewidth wi of the ring, is constant throughout the circumference. Themajor axis of the ellipse is parallel to the direction of theastigmatism. Since the image in FIG. 8 is again an image formed in thetop right angle of the image plane, the major axis of the ellipseextends under 45°. The astigmatism of the projection lens system hasbeen obtained artificially by deliberately displacing a movable lenselement of this system to some extent with respect to its nominalposition.

[0105] Generally, the points of the contour lines may be represented bythe series:${\overset{\sim}{\sum\limits_{m = 0}}{{{R_{m}(r)} \cdot \cos}\quad \left( {m\quad \theta} \right)}} + {{R_{m}^{\prime}(r)}\sin \quad \left( {m\quad \theta} \right)}$

[0106] in which R_(m)(r) is the nominal distance of the relevant pointto the center of the image, r indicates whether the relevant point isassociated with either the inner contour or the outer contour, cos(mΘ)and sin(mΘ)) is the angle dependence of the real distance between therelevant point and the center and m is determined by the type ofaberration.

[0107] For spherical aberration, m=0. This aberration is not dependenton the angle Θ, and an image formed with an imaging system havingspherical aberration is rotationally symmetrical around the optical axisof the imaging system, i.e. around the Z axis in FIG. 1. The change ofthe image due to spherical aberration is dependent on the position alongthe Z axis.

[0108] For the comatic aberration, m=1. An image formed with an imagingsystem having this aberration has a single axis of symmetry, in theexample of FIG. 7 the axis under 45° along which the circles aredisplaced with respect to each other.

[0109] For the astigmatic aberration, m=2. When this aberration occurs,the formed, elliptic image has two axes of symmetry, namely the majoraxis and the minor axis of the ellipse. In the example of FIG. 8, theseare the axes shown under 45° and an axis perpendicular thereto.

[0110] For the three-leaf, or three-point, aberration, m=3. When thisaberration occurs, the image shows three axes of a symmetry. The imageof FIG. 7 does not only have comatic aberration but also a smallthree-point aberration. An image having a larger three-point aberrationis shown in FIG. 9.

[0111] The description has hitherto been based on a single test pattern.However, a test mask may have a large number of test patterns, forexample 121, so that the aberrations can be measured at an equally largenumber of positions in the image field of the projection lens system. Inpractice, not all of these test patterns, but a smaller number will beused, for example 21, in which these test patterns are located at suchpositions that most information about aberrations can be obtainedtherefrom. Since the test patterns are so small, they may also beprovided in a production mask, i.e. a mask with an IC pattern, withoutthis being at the expense of the details of the relevant IC pattern.Then it is not necessary to manufacture separate test masks and toexchange masks for measuring aberrations.

[0112] For performing the novel method by means of a lithographicprojection apparatus, the projection beam preferably has a small beamcross-section at the location of the mask so that a maximal quantity ofprojection radiation is concentrated on the test pattern and a clearimage is obtained. Novel generations of lithographic projectionapparatuses have special illumination systems which provide, inter alia,the possibility of adapting the cross-section of the projection beam,with the total radiation energy of the beam being maintained.

[0113] Such an illumination system is described, for example, in thearticle: “Photolithography using the AERIAL illuminator in a variable NAwafer stepper” SPIE Vol. 2726, Optical Microlithography IX, 13-15 March1996, pp. 54-70. The ratio between the cross-section of the projectionbeam and the pupil cross-section is denoted by σ, or degree ofcoherence. For projecting the mask pattern, σ values of between 1 and0.3 are currently used. In accordance with the invention, such alithographic apparatus can be made eminently suitable for performing thenovel method of measuring aberrations if the means for limiting the beamcross-section are implemented in such a way that the C values can be setat the order of 0.2 or less. These means can be obtained by adapting thebeam-limiting means already present in the lithographic apparatus insuch a way that the cross-section of the projection beam can be madeconsiderably smaller than the beam cross-section which is used forprojecting the mask pattern on the substrate. This further reduction ofthe beam cross-section can then be realized while maintaining the totalenergy in the beam. For the aberration measurements, it is alternativelypossible to arrange an extra diaphragm in the radiation path between theradiation source and the mask holder, the aperture of said diaphragmbeing adjustable in such a way that σ values of between 1 and, forexample, 0.1 can be adjusted.

[0114] The use of the invention in a stepping lithographic apparatus hasbeen described hereinbefore, i.e. in an apparatus in which the wholemask pattern is illuminated and imaged in a first IC area, andsubsequently the mask pattern and the substrate are moved with respectto each other until a subsequent IC area is positioned under the maskpattern and the projection system, hence one step is made, whereafterthis IC area is illuminated with the mask pattern, another step is madeagain, and so forth until the mask pattern has been imaged on all ICareas of the substrate. To alleviate the requirements of a large NA anda large image field imposed on the projection lens system and/or toincrease the resolution and the image field of the apparatus, astep-and-scanning apparatus is preferably used. In this apparatus, amask pattern is not imaged as a whole in one step, but the mask patternis illuminated by a beam having a narrow, rectangular or circularlysegment-shaped beam cross-section, and the mask pattern and thesubstrate are moved synchronously with respect to the system, whiletaking the magnification of the projection system into account, so thatall sub-areas of the mask pattern are consecutively imaged oncorresponding sub-areas of the substrate. Since the cross-section of theprojection beam in one direction, for example, the X direction, isalready small in such an apparatus, only the beam cross-section in theother direction, for example, the Y direction should be decreased so asto obtain an optimal illumination for the novel method.

[0115] FIGS. 10-25 show a number of examples of measuring resultsobtained by means of the method according to the invention.

[0116] FIGS. 10-14 relate to spherical aberration. As already noted, theannular image remains rotationally symmetrical when this aberrationoccurs, but the width wi of the ring is dependent on the extent ofdefocusing. In the experiment performed, a spherical aberration wasintroduced artificially by readjusting the height, the Z position, ofthe mask table by 40 μm with respect to the nominal height. FIG. 10shows the annular images obtained by readjusting the focus of theprojection lens system from −0.3 μm to +0.3 μm with respect to thenominal focus. FIG. 11 shows the then occurring change of the width ofthe ring in a graphic form. In this Figure, the defocusing DEF isplotted in μm on the horizontal axis and the ring width wi is plotted onthe vertical axis. As is shown in FIG. 11, the ring width at nominalfocus setting has increased from the above-mentioned 80 nm toapproximately 130 nm, while a ring width of 80 nm is obtained at adefocusing of 0.4 μm.

[0117]FIG. 12 shows the shape and the location in the XY plane of anannular image in the best focus position. The origin of the XY system ofco-ordinates is located on the optical axis of the projection system.FIG. 13 shows the Fourier analysis data of this image. The Fourier termsFT expressed in frequencies of the angle Θ are plotted on the horizontalaxis. The Fourier term at the position 1 represents coma which isproportional to cosΘ, that at position 2 represents astigmatism which isproportional to cos2Θ, that at position 3 represents three-pointaberration which is proportional to cos3Θ, and that at positions 4, 5and 6 represents other aberrations which are negligibly small for theexample given. The amplitudes of the deviations of the circle areplotted in nm on the vertical axis. For the example shown in FIG. 12,there is some coma at an angle Θ=124°, some astigmatism at Θ=178° andsome three-point aberration at Θ=−2°.

[0118] The three-dimensional FIG. 14 shows an example of the variationthroughout the image field, in this example 20×20 mm large, of aspherical aberration. The X and Y positions in the image field areplotted on the axes of the base plane and the spherical aberration isplotted on the vertical axis. This aberration is expressed in the numberof nm change of the ring width wi per μm offset of the focus. Theaverage spherical aberration across the image field is equal to −85nm/μm in this example.

[0119] FIGS. 15-18 relate to a comatic aberration which has beenintroduced artificially by imaging, the test object with radiation whosewavelength is 40 pm larger than the nominal wavelength, i.e. thewavelength for which the projection system has been designed. FIG. 15shows the annular images 40-48 which are then formed at differentpositions in the image field. As already noted, the inner contour andthe outer contour are offset with respect to each other when comaoccurs, so that these contours are no longer centered with respect toeach other. The coma is relatively small in the center of the imagefield, as is shown by the central image 40. Upon a displacement from thecenter, the coma increases, while the direction of the coma coincideswith the direction of the displacement. The coma directions areapproximately +45°, +135°, −135° and −45° for the images 45, 46, 47 and48, respectively.

[0120] The coma is not only dependent on the position in the image fieldbut also on the extent to which the imaging, beam is focused on thephotoresist layer. If at a fixed position in the image field scanning,through focus takes place, then the coma has a parabolic variation as afunction of the defocusing, with the smallest coma occurring if thefocusing is optimal. FIG. 16 shows a magnification of the image 48 forthe best focus condition.

[0121]FIG. 17 shows the Fourier graph associated with the image of FIG.16. It is apparent from FIG. 17 that the direction of the coma is −48°and its amplitude is 30 nm. The projection system with which this imageis made also has an astigmatism of approximately 7 nm at an angle Θ of118° and a three-point aberration of approximately 5 nm at an angle Θ of17°.

[0122]FIG. 18 shows a coma chart obtained by imaging, the test patternat 21 different positions in the image field, the XY plane. Thedirection of the coma at a measured position is indicated by thedirection of the arrow shown at that position and the size of the comais indicated by the underlined number near this arrow. Each number inFIG. 18 is the average of the coma numbers associated with the relevantfield position and obtained by scanning through focus. The average comathroughout the image field of the example shown in FIG. 18 is 18 nm.

[0123] FIGS. 19-22 relate to an astigmatic aberration. Instead of asingle focal point, an imaging system having astigmatism has a first anda second, astigmatic, focal line, which focal lines are perpendicular toeach other. The length of these focal lines is dependent on the positionalong the optical axis of the imaging, system. In the position where thebeam has its narrowest constriction, the focal lines are equally longand the image is circular. At positions located before the position ofnarrowest constriction, the first focal line is longer than the second,and the image is elliptical, with the major axis of the ellipseextending in the direction of the first focal line. At positions locatedbehind the position of the narrowest constriction, the second focal lineis longer than the first, and the image is elliptical, with the majoraxis of the ellipse extending in the direction of the second focal line.To determine the astigmatic aberration of a lens system, it is necessaryto scan through focus. In accordance with the novel method, theastigmatism is detected by determining the change of the second harmonic(2Θ) as a function of the defocusing. This astigmatism is expressed innm per μm defocusing.

[0124]FIG. 19 shows images 50-58 which are formed at nine differentpositions in the image field by a projection system having astigmatism.This astigmatism has been introduced artificially by displacing amovable lens element of the projection system by 40 μm with respect toits nominal position. In the center of the image field, the astigmatismis relatively small as is shown by the central image 50. Upon adisplacement from the center, the astigmatism increases, with thedirection of the astigmatism coinciding with the direction of thedisplacement. The directions of astigmatism are approximately +45°,+135°, −135° and −45° for the images 55, 56, 57 and 58, respectively.

[0125]FIG. 20 shows a magnification of the image 58 and FIG. 21 showsthe associated Fourier graph. It is apparent from the latter Figure thatthe direction of the astigmatism is 136° and its size is approximately18 nm/μm. The projection system with which this image is formed also hasa coma of 11 nm at an angle Θ of 51°, a three-point aberration of 4 nmat an angle Θ of 11° and a four-point aberration of 6 nm, proportionalto cos4Θ, at an angle Θ of 3°. An image formed by a projection lenssystem with four-point aberration has four axes of symmetry.

[0126]FIG. 22 shows an astigmatism chart obtained by forming images ofthe test pattern at 21 different positions of the image field, the XYplane. The direction of astigmatism at a measured position is indicatedby the direction of the arrow at that position, and the amount of theastigmatism is indicated by the underlined number at that position. Eachnumber in FIG. 22 is the average of the astigmatic numbers associatedwith the relevant position and obtained by scanning through focus. Forthe given example, the average astigmatism throughout the image field,i.e. the average of the numbers of FIG. 22 is 31.1 nm.

[0127] An example of measured three-point aberrations is shown in thethree-dimensional FIG. 23. The X and Y positions in the image field areplotted along the axes of the base plane in this Figure and the size ofthe aberration is plotted in run on the vertical axis. Also thisaberration is maximal at the angles of the image field. The aberrationis relatively small; the average value of this aberration in thisexample is 4.7 nm.

[0128]FIGS. 24 and 25 illustrate that simultaneously occurringaberrations of different types can be measured separately by means ofthe method according to the invention. FIG. 24 shows a coma chart whichis similar to that of FIG. 18. Not only first arrows representing thepure coma are shown at the 21 different positions in the image field,but also second arrows representing the measured coma in the presence ofspherical aberration, and third arrows representing the measured coma inthe presence of astigmatism. It is apparent from this Figure that themeasured coma size and direction generally changes to only a smallextent when said two other aberrations occur.

[0129]FIG. 25 shows an astigmatism chart which is similar to that ofFIG. 22. Not only first arrows representing the pure astigmatism areshown at the 21 different positions in the image field, but also secondarrows representing the astigmatism in the presence of sphericalaberration, and third arrows representing the astigmatism in thepresence of coma. It is apparent from this Figure that the measured sizeand direction of the astigmatism generally changes to only a smallextent when spherical aberration and coma occur simultaneously.

[0130] The circular phase structure(s) cover(s) only a very small partof the mask surface area. If an entirely transparent test mask is used,the radiation passed by the mask outside the area of the phase structuremay have the effect of interference radiation and reduce the quality ofthe image of the phase structure. To prevent this, a test mask ispreferably used in which only the circular phase structure, furtherreferred to as the figure, and a relatively small area around it,hereinafter referred to as figure area, are transparent, while the restof the mask, hereinafter referred to as outer area, has been madeopaque, for example by coating it with chromium. FIG. 26 shows a part ofa test mask TM having a circular phase structure, or area, denoted bythe reference numeral 22 again. The transparent figure area around thecircle 22 is denoted by the reference numeral 80. This area consists oftransparent mask material (20 in FIG. 3b). Outside the figure area, themask is coated with a chromium layer 82.

[0131] To achieve that a scanning electron microscope, or anotherscanning detection device, can easily find the small image of the FIG.22, a recognition mark 84 is provided in the test mask and in the outerarea of each phase pattern, as is shown in FIG. 26. This mark, which isformed by an F-shaped opening in the chromium layer in the exampleshown, may be an arbitrary mark, provided that it has details extendingin the X direction as well as details extending in the Y direction. Asis shown by FIG. 26, the strips extending in the X direction and thestrips extending in the Y direction of the recognition mark areconsiderably larger than the FIG. 22 so that this mark is more easilyobservable and is suitable for navigation of the detection device. Assoon as this mark has been observed, the detection device can bedirected within the area on the substrate which corresponds to the outerarea 82 of the test mask to the image of the figure area 80 and startsearching the image of the FIG. 22 located within this area. Opaque,chromium-coated strips 86 in the X direction and strips 88 in the Ydirection may be present within the figure area 80 so as to simplify thenavigation of the detection device within the area on the substratecorresponding to the figure area 80.

[0132] Further information may be provided, as is denoted by thereference numeral 90, in each outer area 82 of the test mask. In thisexample, the information relates to the diameter of the imaged ring (din FIG. 4) chosen for the relevant area 82. This information may alsobe, for example, position information and indicate the X and Yco-ordinates of the relevant figure area 80 on the test mask. Furtherinformation which may be useful for performing the method may also beprovided in the recognition mark 84.

[0133] Since the marks 84 and 90 have relatively large details, thesedetails will always be imaged in such a way that they are stillreasonably recognizable for the scanning detection device, even if theimaging circumstances are not ideal, for example, if the quantity ofillumination used is not optimal. If, for example, a too small quantityof illumination were used, the quality of the image of the phase FIG. 22would be reduced to such an extent that the method can no longer be usedsatisfactorily. By observing the mark 84 and/or 90, the cause of thepoor image quality can be ascertained, so that the circumstances can beadapted thereto in such a way that a usable image of the phase patternis as yet obtained and the method can still be used.

[0134] It has hitherto been assumed that the phase FIG. 22 is formed byan area which is located higher or lower than the rest of the plate ortest mask 20. The phase figure may, however, also consist of an areahaving a different refractive index than the rest of the plate.

[0135] Such an area introduces also a phase jump in a beam passingthrough the plate. If a reflecting production mask is used in thelithographic apparatus, and if the novel method is performed with areflecting test mask, the FIG. 22 and the figure area 80 will have to betransparent to this test mask so as to cause this FIG. 22 to be activeas a phase structure with a deviating refractive index. To reflect theimaging beam which has passed through the test mask at the location ofthe FIG. 22 and the figure area 80, the test mask may be provided withreflecting means at the relevant locations.

[0136] The text hereinbefore only describes measurements on a projectionlens system for a lithographic apparatus. However, the projection systemfor such an apparatus may also be a mirror projection system. Such aprojection system must be used if EUV radiation is used as projectionradiation. EUV, or extreme ultraviolet, radiation is understood to meanradiation at a wavelength in the range of several nm to several tens ofnm. This radiation is also referred to as soft X-ray radiation. The useof EUV radiation provides the great advantage that extremely smalldetails, of the order of 0.1 μm or less, can be imaged satisfactorily.In other words, an imaging system in which EUV radiation is used has avery high resolution without the NA of the system having to be extremelylarge so that also the depth of focus of the system still has areasonably large value. Since no suitable material which is sufficientlytransparent and suitable for making lenses is available for EUVradiation, a mirror projection system instead of a conventionalprojection lens system must be used for imaging a mask pattern on thesubstrate. Different embodiments of such mirror projection systems areknown, which may comprise three to six mirrors. As the number of mirrorsincreases, the quality of the image is enhanced, but due to reflectionlosses, this is at the expense of the quantity of radiation on thesubstrate. A mirror projection system with six mirrors is described in,for example EP-A 0 779 528.

[0137]FIG. 27 shows an embodiment of another type of mirror projectionsystem with six mirrors for a step-and-scanning lithographic projectionapparatus having an NA (at the image side) of the order of 0.20, amagnification M of 0.25, a circular segment-shaped image field having awidth of 1.5 mm and a relatively large free working distance fwd. Theapparatus comprises an illumination unit 60, shown diagrammatically,accommodating an EUV radiation source and an optical system for forminga projection beam PB whose cross-section has the shape of a circularsegment. As is shown in the Figure, the illumination unit may bepositioned close to the substrate table WT and the imaging section 69,70 of the projection system, so that the projection beam PB can enterthe projection column closely along these elements. The mask MA′ to beimaged, which is a reflective mask in this example, is arranged in amask holder MH which forms part of a mask table MT by means of which themask can be moved in the scanning direction 62 and possibly in adirection perpendicular to the scanning direction, such that all areasof the mask pattern can be arranged under the illumination spot formedby the projection beam PB. The mask holder and mask table are shown onlydiagrammatically and may be implemented in various ways. The substrate Wis arranged on a substrate holder which is supported by a substratetable WT. This table may move the substrate in the scanning direction,the X direction, but also in the Y direction perpendicular thereto. Inthis embodiment, the mask and the substrate move in the same directionduring scanning. The substrate table is supported by a block 64.

[0138] The projection beam reflected by the reflective mask MA isincident on a first mirror 65 which is concave. This mirror reflects thebeam as a converging beam to a second mirror 66 which is slightlyconcave. The mirror 66 reflects the beam as a more strongly convergingbeam to a third mirror 67. This mirror is convex and reflects the beamas a slightly diverging beam to the fourth mirror 68. This mirror isconcave and reflects the beam as a converging beam to the fifth mirror69 which is convex and reflects the beam as a diverging beam to thesixth mirror 70. This mirror is concave and focuses on the photoresistlayer PR provided on the substrate W. The mirrors 65, 66, 67 and 68jointly form an intermediate image of the mask, and the mirrors 69 and70 produce the desired telecentric image of this intermediate image onthe photoresist layer PR.

[0139] Also the mirror projection system described above and otherprojection systems may have said aberrations: spherical aberration,coma, astigmatism, three-point aberration and possible furtheraberrations, and also these aberrations can be measured accurately andreliably by means of the novel method. In the EUV lithography, areflective mask is preferably used, inter alia, because such a mask canbe better supported than a transmissive mask. The test pattern requiredfor the novel method in a reflective test mask or production mask mustthen have a depth which is equal to one quarter of the wavelength if thesurrounding medium is air. This implies that a depth of 3.25 nm isnecessary for the wavelength of 13 nm preferred in EUV lithography,which depth is very small. In that case, the FIG. 22 with the phasestructure may also consist of an area in the plate or test mask 20having a different refractive index than the rest of this plate.

[0140] As is apparent from the examples described above, the aberrationsare relatively small for the measured lithographic projection systems.In practice, it is therefore as yet unnecessary to measure higher-orderaberrations. However, as is apparent from the Fourier graphs of FIGS.13, 17 and 21, the novel method is also suitable for measuring thesehigher-order aberrations.

[0141] The fact that the invention has been described with reference tothe measurements on a projection lens system or a mirror projectionsystem for a lithographic projection apparatus does not mean that itsapplication is limited thereto. The invention may be used wherever theaberrations of an imaging system must be measured independently of eachother and with great accuracy and reliability. An example of such animaging system is a space telescope. When using the novel method in alithographic projection apparatus, an optimal use is, however, made ofthe fact that this apparatus itself is already intended for imagingpatterns on substrates and that the imaging and servosystems of thisapparatus may also be used for performing the novel method. Moreover,possible means desired for performing the method, such as said extradiaphragm, can easily be arranged in the apparatus.

1. A method of detecting aberrations of an optical imaging system,comprising the steps of arranging a test object in the object plane ofthe system; providing a photoresist layer in the image plane of thesystem; imaging the test object by means of the system and an imagingbeam; developing the photoresist layer, and detecting the developedimage by means of a scanning detection device having a resolution whichis considerably larger than that of the imaging system, characterized inthat use is made of a test object which comprises at least one closedsingle figure having a phase structure, and in that the image of thisfigure observed by the scanning detection device is subjected to animage analysis in order to ascertain at least one of different types ofchanges of shape in the image of the single figure, each type of shapechange being indicative of a given kind of aberration.
 2. A method asclaimed in claim 1 , characterized in that a scanning electronmicroscope is used as a scanning detection device.
 3. A method asclaimed in claim 1 or 2 , characterized in that the image analysismethod used comprises a Fourier analysis.
 4. A method as claimed inclaim 1 , 2 or 3, characterized in that every single figure isconstituted by an area in a plate located at a different height than therest of said plate.
 5. A method as claimed in claim 4 , characterized inthat the height difference between the area of the single figure and therest of the plate is such that a phase difference of 180° is introducedin the imaging beam.
 6. A method as claimed in claim 4 or 5 ,characterized in that the diameter of the area is proportional toλ/(NA.M), in which λ is the wavelength of the imaging beam, NA is thenumerical aperture of the imaging system at the image side and M is themagnification of the imaging system.
 7. A method as claimed in claim 1 ,2 , 3, 4, 5 or 6 of detecting aberrations of a projection system in alithographic projection apparatus intended to project a mask pattern,present in a production mask, on a production substrate provided with aphotoresist layer, characterized in that a mask having at least a singlefigure with a phase structure is arranged at the position of theproduction mask in the projection apparatus, and in that a photoresistlayer with a support is provided at the position of a productionsubstrate.
 8. A method as claimed in claim 7 , characterized in that useis made of an empty test mask having at least a single area with a phasestructure.
 9. A system for performing the method as claimed in any oneof the preceding claims, which system is constituted by the combinationof: an apparatus of which the imaging system forms part; a test objecthaving at least a single figure with a phase structure; a photoresistlayer in which the test object is imaged; a scanning detection devicefor scanning at least a test object image formed and developed in thephotoresist layer, and an image processor, coupled to the scanningdetection device, for storing and analyzing the observed images,characterized in that the image processor comprises analysis means fordetecting at least one of different types of shape changes in the formedimage of the single figure.
 10. A lithographic projection apparatus forimaging a mask pattern, present in a mask, on a substrate, whichapparatus comprises an illumination unit for supplying a projectionbeam, a mask holder for accommodating the mask, a substrate holder foraccommodating the substrate, and a projection system arranged betweenthe mask holder and the substrate holder, said apparatus being suitablefor performing the method as claimed in any one of claims 1-8,characterized in that, in the implementation of the method, theprojection beam is used as an imaging beam, and in that the illuminationunit comprises means for reducing the diameter of the projection beamcross-section for the method to a value which is smaller than thediameter of the projection beam cross-section during projection of themask pattern on the substrate.
 11. A test object having one or more ofsaid characteristic features as claimed in any one of claims 1-8,relating to the test object.
 12. A test object as claimed in claim 11 ,implemented as a test mask, characterized in that each closed figure issurrounded by a figure area which is formed by a part of the masksurface area, which figure area has the same transmission or reflectioncoefficient as the figure.
 13. A test object as claimed in claim 11 inwhich each figure area is surrounded by an outer area which is formed bya part of the mask surface area, characterized in that each outer areais provided with a recognition mark having the same transmission orreflection coefficient as the figure and the figure area.
 14. A testobject as claimed in claim 13 , characterized in that each figure areais provided with navigation marks for the detection apparatus, whichnavigation marks have a different transmission or reflection coefficientthan the figure and the figure area.
 15. A test object as claimed inclaim 13 or 14 , characterized in that each outer area is provided witha further mark comprising information about the figure and/or itsposition on the mask surface area, said further mark having the sametransmission or reflection coefficient as the figure and the figurearea.
 16. A test object as claimed in claim 12 , 13 , 14 or 15,characterized in that the test mask is transparent, and in that thesurface area of this mask outside the figure and the figure area andoutside the recognition mark and the further mark, if these are present,is provided with a radiation-obstructive layer.
 17. A test object asclaimed in claim 12 , 13 , 14 or 15, characterized in that the test maskis reflective, and in that the surface area of this mask outside thefigure and the figure area and outside the recognition mark and thefurther mark, if these are present, is provided with aradiation-absorbing layer.